This invention relates to electrodeposited coatings, and, more particularly, to such a coating incorporating nickel, tungsten, and boron and that has high hardness but low residual stress.
Coatings are widely used to protect substrates in wear-inducing and/or corrosive environments. A substrate is formed to the required shape and dimensions, and a coating is placed onto the substrate. The material of the substrate is chosen to provide the required combination of strength, toughness, ductility and other properties for the structure, and the coating is chosen to protect the surface of the substrate against hostile environments. The coating must also adhere well to the substrate, and accordingly should have a low residual internal stress that does not cause the coating to delaminate from the substrate.
Amorphous and microcrystalline materials offer promise for use as protective coatings. An amorphous material has no long-range or short-range crystallographic order, and, and therefore, no grain boundaries that can preferentially erode or corrode. Microcrystalline (including nanocrystalline) materials have very small grains, but have been observed to have excellent erosion and corrosion resistance. Certain types of amorphous and microcrystalline materials also exhibit extremely high hardnesses, making them ideal candidates for protective coatings.
To utilize amorphous and microcrystalline materials as protective coatings, techniques must be available to apply the coatings to the surfaces to be protected. It has been known, for example, to deposit molten metals, which naturally have no long-range or short-range order, against a surface so that they solidify rapidly and, in some cases, remain in the disordered state. This rapid solidification approach is practiced for some applications, but not for others such as the coating, of the insides of tubes. High rate solidification also is, by its nature, somewhat uncontrolled, and can lead to variations in thickness and structure. High rate solidification can be accomplished by splatting a molten metal against a surface or by melting a solid surface coating and causing it to freeze rapidly.
Another approach offers the capability to form amorphous and microcrystalline coatings in a more controlled fashion, which can also be used for applications such as the coating of interior regions. In this technique, an electrodeposition bath is prepared from compounds of the elements to be codeposited. A metal is electrodeposited from this bath onto a cathode, depositing as a thin protective coating.
One such electrodeposition approach is described in U.S. Pat. No. 4,529,668. According to this process, a boron-containing amorphous alloy is deposited from a bath containing, for example, ions of tungsten, cobalt, and boron. The resulting tungsten-cobalt-boron compound is amorphous with a high hardness and wear resistance. It may be deposited on exterior and interior surfaces, uniformly and with great control.
One important consideration of any candidate for use in production operations is its economics, and in particular the cost of depositing coatings of preselected thicknesses. When coatings are deposited, the economics can often be expressed in terms of the rate of coating deposition. The slower the rate, the longer the time to deposit a coating of some preselected thickness. The process of the '668 patent deposits its amorphous coating at a rate of about 0.001-0.003 inches in eight hours of deposition. This rate is fully acceptable for many applications. However, this deposition rate may be too slow for other coating requirements.
Thus, there is always an ongoing need for techniques to produce desirable coatings at higher deposition rates. The present invention fulfills this need, and further provides related advantages.